The present invention relates to Melamine based Mannish compounds, a process for obtaining the same, precondensation products and their uses.
Monomers based on melamines and phenols are interesting building blocks for the synthesis of resins (Ullmans encyclopedia of Industrial Chemistry, 2005, 6th ed).
Melamine-based resins are usually obtained from polymerization of melamines and aldehydes, in particular formaldehyde. The degree of polymerisation is adjustable by the applied reaction conditions, such as temperature, pH-value, catalysts or fillers. Melamine pre-condensates can be etherified and used as surface coating resins. Melamine-formaldehyde resins are mixable with alkyd resins, acrylates and epoxide resins. In the presence of saturated polyesters they cure at room temperature. Melamine resins are widely applicable in particular for the production of laminates, electrically isolating material, glues for paper or the wood industry and others.
Phenol based resins are obtained by condensation of phenols and formaldehydes. Two types are distinguishable: novolak and resol. Novolak is synthesized in an acidic reaction media with a stoichiometric deficiency of formaldehyde. The non-hardened products are crosslinked in the presence of hardeners. Resol on the other side are obtained in an alkali- or basic media with an excess of formaldehyde. Novolak and resol have a good stability, stiffness, stress crack resistance and hardness. They are mainly used as glues for wood and wood fibres, for insulating material, hard paper or as binders. A drawback of phenol based resins is their yellowish till brownish colour and their instability in the presence of strong acids and bases.
Different attempts have been carried out to combine the properties of melamine and phenole based resins. As one approach different reactions conditions for a combined condensation of phenol, melamine and formaldehyde were tested (Braun and Ritzert, Angewandte Makromol. Chemie, 1984, 125: 9-26). By varying pH-value and the molar ratio the authors were able to show that neither under acidic conditions nor basic conditions a co-condensation of melamine, phenol and formaldehyde occurred. This is due to the different pH-depenndet reaction mechanism: the reactivity of phenol towards formaldehyde is highest in the basic milieu whereas the reactivity of melamine is highest in the acidic reaction milieu. Any products obtained by a simultaneous reaction of phenol, melamine and formaldehyde have to interpreted as interpenetrating networks of phenol resins and melamine resins (Braun and Ritzert, Angewandte Makromol. Chemie, 1984, 125: 27-36).
The Mannich reaction [Mannich C, 1917, Arch Pharm. 255, 261-276] is well known. The Mannich-reaction comprises the condensation of an aldehyde, preferably formaldehyde, with ammonia, a primary or secondary amine under formation of an imminium-ion which undergoes a reaction with an enol-forming carbonyl compound like an aldehyde or ketone. The reaction mechanism is similar to an aldol condensation whereby the enol attacks a C═N double bond instead of a carbonyl bond.
Bujnowski et al. (Akrivoc, 2008, 106-114) describes a Mannich reaction of phenol, formaldehyde and hexahydro-1,3,5-triazine. The reactivity of this hexahydrotriazine is close to that of an aliphatic amine, so that it reacts in a Mannich-type reaction.
Aminotriazine structures that are similar to melamines have been used for the synthesis of compounds comprising a phenol derivative and a triazine (WO 03/032903). However, cyanuric chloride has been used as starting material to synthesize the aminotriazine structures.
Several attempts haven been made in the past to use melamine in a Mannich reaction.
For instance suggests Rima at el. (J. of Food Composition and Analysis, 2009, 22:689-693) a reaction of melamine with uranine in formaldehyde solution in a Mannich-type manner. Interestingly, the hypothesized Mannich-product is formed without the addition of any acid.
Maciejewski et al. (Polymer Bulletin, 2002, 48:251-259) postulates the formation of a mixture of hydroxyphenylmethylmelamines based on the Mannich-reaction of methylolated melamine and an excess of phenol in the presence of stoichometric amounts of hydrochloric acid. It is suggested that the obtained product comprises five phenol units per one melamine molecule.
Even though the above references suggest the possibility of applying melamine in a Mannich-type reaction, so far no further studies have been carried out for using in particular substituted melamines as educts in a Mannich-type reaction and the further use of the obtained products for obtaining pre-condensates.